Casting steel strip

ABSTRACT

A method of producing steel strip by continuous casting and a twin roll caster in which a scrap receptacle is provided for collecting scrap. The scrap receptacle is positioned in a scrap receiving position and may be changed out as desired with a replacement receptacle with little or no interruption to casting operations. In the receiving position the scrap receptacle may form a part of a sealed casting enclosure. The scrap receptacle may be monitored by a sensor to help determine when it is desirable to replace the receptacle with the replacement receptacle.

BACKGROUND AND SUMMARY OF THE INVENTION

[0001] This invention relates to continuous casting of steel strip in a strip caster, particularly a twin roll caster.

[0002] In a twin roll caster, molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the casting rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle through one or more smaller vessels to a metal delivery nozzle located above the nip so as to direct the molten metal into the nip between the rolls, forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the casting rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electro magnetic barriers have also been proposed.

[0003] When casting steel strip in a twin roll caster, the strip leaves the nip at very high temperatures, of the order of 1400° C., and if exposed to normal atmosphere, it suffers very rapid scaling due to oxidation at such high temperatures. It has therefore been proposed to provide a sealed enclosure beneath the casting rolls to receive the hot strip and through which the strip passes away from the strip caster, the enclosure containing an atmosphere which inhibits oxidation of the strip. The oxidation inhibiting atmosphere may be created by injecting a non-oxidizing gas, for example, an inert gas such as argon or nitrogen, or combustion exhaust gases which may be reducing gases. Alternatively, the enclosure may be sealed against ingress of oxygen containing atmosphere during operation of the strip caster and the oxygen content of the atmosphere within the enclosure reduced during an initial phase of casting by allowing oxidation of the strip to extract oxygen from the sealed enclosure as disclosed in U.S. Pat. Nos. 5,762,126 and 5,960,855.

[0004] U.S. Pat. No. 5,960,855 discloses a twin roll caster with a sealed enclosure to receive the strip in which a bottom part of the sealed enclosure is formed to serve as a moveable receptacle for scrap produced at any stage of the casting process. For example, on start up an initial length of strip may need to be discharged as scrap to form a clean end to be fed to downstream equipment. If severe defects arise during a casting run, it may also be necessary to discharge lengths of strip to scrap at various times through a casting run. When the scrap receiving receptacle becomes full, it must be removed to allow the scrap to be discharged. Normally, this would require the casting run to be terminated with a consequent loss of casting time and a need to reheat a charge of molten feed metal and all of the refractories in the metal delivery system. This interruption in casting also adversely effects productivity and adds to the production costs. By the present invention, it is possible to replace the scrap receiving receptacle with an empty replacement receptacle during a casting run and to proceed with casting without the need for reheating of metal or refractories.

[0005] The invention provides a method of producing thin cast steel strip by continuous casting comprising the steps of:

[0006] a) assembling a pair of cooled casting rolls having a nip between them;

[0007] b) introducing molten steel between the pair of casting rolls through a metal delivery system configured to a casting pool of molten metal supported on casting surfaces of the casting rolls;

[0008] c) counter-rotating the casting rolls to form metal shells on the casting surfaces of the casting rolls;

[0009] d) forming solidified thin steel strip through the nip between the casting rolls from said formed metal shells;

[0010] e) discharging strip into a first scrap receptacle positioned in a scrap receiving position;

[0011] f) replacing the first scrap receptacle with a second scrap receptacle by the steps comprising:

[0012] (i) optionally, interrupting the flow of molten steel to the metal delivery system,

[0013] (ii) moving the first scrap receptacle away from the scrap receiving position,

[0014] (iii) positioning a second scrap receptacle into the scrap receiving position, and

[0015] (iv) continuing or resuming the flow of molten steel to the metal delivery system to cast steel strip.

[0016] The replacement of the scrap receptacle can be accomplished with little or no interruption in the casting of steel strip. The casting operation can be continued while the scrap receptacles are changed out by minimizing the time in changing the scrap receptacles. In such embodiments, the flow of molten metal to the metal delivery system is not interrupted during the replacement of the first scrap receptacle with a second replacement scrap. In such embodiments, for example, the scrap receptacle in use and the replacement scrap receptacle may be positioned in tandem. The flow of metal to the metal delivery system is continued during the change out of the first scrap receptacle by the second scrap receptacle.

[0017] Alternatively, the scrap receptacle change may take advantage of the molten metal in the metal delivery system and the casting pool to minimize the interruption in the casting operation during the scrap receptacle change out. Specifically, molten steel flow the metal delivery system, namely, a delivery vessel and a delivery nozzle forming the metal delivery system, and the casting pool contains typically at least about 250 pounds of molten metal. Casting of steel strip may be continued while molten metal is draining from the metal delivery system and casting pool, while replacement of the scrap receptacle is at least started. To minimize the interruption in casting in certain embodiments, the initial lowering movement, breaking the seal with the enclosure wall, and the lateral movement of the scrap receptacle may be initiated prior to the time of interruption of the flow of molten steel to the metal delivery system.

[0018] In order to ensure that the refractores in the metal delivery system do not suffer thermal shock on resumption of casting and to inhibit premature solidification of molten steel in the flow passages of the metal delivery system, it is desirable that the time interval between interruption and resumption of the flow of molten metal to the metal delivery system be no more than about 5 minutes. More specifically, the operation may be carried to reduce the scrap receptacle change out time interval to about 240 seconds or less. As noted above, the change out of the scrap receptacle can be accomplished in some embodiments without any interruption in the casting of strip. Alternatively, when the flow of molten metal to the metal delivery system is interrupted, casting of the strip can be continued while the molten metal is draining from the metal delivery system and casting pool during the change out with scrap receptacle moved away and a replacement scrap receptacle moved into that scrap receiving position.

[0019] In any event, the scrap receptacle forms a seal with an enclosure wall of an enclosure receiving the cast strip delivered from the nip. The seal needs to be broken between the scrap receptacle and the enclosure wall to change out the scrap receptacle. In some embodiments, the movement of the scrap receptacle away from the caster may involve an initial lowering of the receptacle to break the seal with the enclosure wall and a lateral movement away from the caster while the casting operation is continuing or before the casting operation is interrupted. If the flow of molten metal to the metal delivery system is interrupted, the flow of molten steel to the delivery system may then be resumed while the replacement scrap receptacle is raised into sealing engagement with the enclosure wall to reestablish the sealed enclosure. Similarly, in order to reduce the time interval during which the flow of molten steel is interrupted, the flow of molten steel may be resumed prior to the initiation of raising movement of the replacement receptacle. An added feature of the timing of breaking the seal between the first scrap receptacle and the enclosure wall is that the atmosphere within the sealed enclosure can be altered by, for example, the addition of controlled amounts of oxygen gas to the atmosphere.

[0020] The invention also may be a twin roll caster apparatus comprising:

[0021] (a) a pair of generally horizontal casting rolls forming a nip between them,

[0022] (b) a metal delivery system capable of delivering molten steel between the casting rolls to form a casting pool of molten steel supported on the casting rolls,

[0023] (c) roll drivers to counter rotate the casting rolls thereby producing a cast strip delivered downwardly from the nip,

[0024] (d) a sealed enclosure to receive the strip delivered from the nip formed with an opening in an enclosure wall, and a first moveable scrap receptacle capable of being positioned in sealing engagement with the enclosure wall around the opening, and

[0025] (e) the enclosure being configured to allow replacement of the first moveable scrap receptacle by a second moveable receptacle during casting operation by breaking the seal of the first scrap receptacle with the enclosure wall and thereafter making the seal by a second replacement scrap receptacle with the enclosure wall.

[0026] The first and second scrap receptacles, or any number of scrap receptacles, thus may be changed out repeatedly during casting operations with little or no interruption of the casting of the strip. Those skilled in the art will appreciate that the use of additional scrap receptacles, for example, third, fourth, fifth and etc., falls within the scope of the invention and may allow reduction of the time period required to effect a switch out of scrap receptacles.

[0027] The invention may further provide a twin roll caster comprising:

[0028] (a) a pair of generally horizontal casting rolls forming a nip between them,

[0029] (b) a metal delivery system comprising a delivery vessel and a nozzle through which to deliver molten steel between the casting rolls above the nip to form a casting pool of molten steel supported on the casting rolls,

[0030] (c) roll drivers to counter-rotate the casting rolls to produce a cast strip delivered downwardly from the nip, and

[0031] (d) a sealed enclosure to receive the strip delivered from the nip having a wall enclosure in sealing engagement with a first moveable scrap receptacle configured to be replaced by a second moveable scrap receptacle during casting operations with little or no interruption in the casting of steel strip.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order that the invention may be more fully explained, an illustrative embodiment will be described in some detail with reference to the accompanying drawings in which:

[0033]FIG. 1 is a vertical cross section through an illustrative twin roll strip caster installation operable in accordance with the present invention;

[0034]FIG. 2 illustrates the metal delivery system for the caster;

[0035]FIG. 3 sealed enclosure to receive the cast strip;

[0036]FIG. 4 is an enlarged vertical cross section through an end part of the twin roll caster;

[0037]FIG. 5 is a cross section taken generally along the line 5-5 in FIG. 4;

[0038]FIG. 6 is a cross section taken generally along the line 6-6 in FIG. 4; and

[0039]FIG. 7 is a plan view of an illustrative scrap receptacle handling system.

DETAILED DESCRIPTION OF THE DRAWINGS

[0040] An illustrative embodiment of a twin roller caster installation comprises a twin roll caster denoted generally as 11 producing a cast steel strip 12 which passes within a sealed enclosure 10 to a guide table 13 which guides the strip to a pinch roll stand 14 through which it exits the sealed enclosure 10.

[0041] After exiting the enclosure 10, the strip may pass through further sealed enclosures and may be subjected to in line hot rolling and cooling treatment forming no part of the present invention.

[0042] Twin roll caster 11 comprises a pair of parallel casting rolls 22 to which molten metal from a ladle 23 is delivered through a tundish 25 to a metal delivery system 24 comprising a delivery distributor vessel 26 and a nozzle 27 which is located above the nip so as to direct the molten metal between the casting rolls to form a casting pool supported on the casting surfaces of the rolls.

[0043] Tundish 25 is fitted with a lid 28. Molten steel is introduced into the tundish from ladle 23 via an outlet nozzle 29 and flows from the tundish through an outlet 31 through an outlet nozzle 32 to the delivery nozzle 27 via the distributor vessel 26. The tundish is fitted with a stopper rod 33 and a slide gate valve 34 to selectively open and close the outlet 31 and effectively control the flow of metal from the tundish to the metal delivery system.

[0044] The casting pool of molten steel supported on the casting rolls is confined at the ends of the rolls by a pair of side closure dams or plates 35 which are applied to stepped ends of the rolls by a pair of thrusters 36 comprising hydraulic cylinder units connected to side plate holders 37.

[0045] The casting rolls 22 are internally water cooled so that shells solidify on the moving roller surfaces and are brought together at the nip between them to produce the solidified strip which is delivered downwardly from the nip between the rolls.

[0046] At the start of a casting operation a short length of imperfect strip is produced as the casting conditions stabilize. After continuous casting is established, the casting rolls are moved apart slightly and then brought together again to cause this leading end of the strip to break away so as to form a clean head end of the following cast strip. The imperfect material drops into a first scrap receptacle 40, which may be moveable, located in a scrap receiving position beneath caster 11 and forming part of the sealed enclosure 10 as described below. At this time, a swinging water-cooled apron 38 which normally hangs downwardly from a pivot 39 to one side of the caster outlet is swung across the caster outlet to guide the clean end of the cast strip onto the guide table 13 which feeds it to the pinch roll stand 14. Apron 38 is then retracted back to its hanging position to allow the strip to hang in a loop beneath the caster before it passes to the guide table where it engages a succession of guide rollers.

[0047] The twin roll caster illustratively may be of the kind which is illustrated in some detail in U.S. Pat. Nos. 5,184,668 and 5,277,243 and reference may be made to those patents for appropriate constructional details which form no part of the present invention.

[0048] Sealed enclosure 10 is formed by a number of separate wall sections which fit together at various seal connections to form a continuous enclosure wall that permits control of the atmosphere within the enclosure. The seal of the enclosure 10 may not be complete, but appropriate to allow control of the atmosphere within the enclosure as described herein. These comprise a wall section 41 which is formed at the twin roll caster to enclose the casting rolls, and an enclosure wall 42, which may extend downwardly as part of the enclosure 10 beneath wall section 41, that has an opening therein The enclosure wall surrounding the opening is in engagement with the upper edges of the scrap receptacle 40, which illustratively is generally in the form of a box, so that the scrap receptacle may be in sealing engagement with the enclosure wall 42.

[0049] Such a seal 43 between the first scrap receptacle 40 and the enclosure wall 42 may be formed by a knife and sand seal which can be established and broken by vertical movement of the scrap receptacle relative to the enclosure wall. More particularly, the upper edge of the scrap receptacle may be formed with an upwardly facing channel 48 which is filled with sand and which receives a knife flange 49 depending downwardly around the opening in the wall enclosure 42. The seal 43 is formed by raising the scrap receptacle to cause the knife flange 49 to penetrate the sand in the channel 48 to establish the seal 43. This seal 43 can be broken by lowering the scrap receptacle 40 from its operative position preparatory to movement away from the scrap receiving position to a scrap discharge position.

[0050] Scrap receptacle 40 is mounted on a carriage 45 fitted with wheels 46 which run on rails 47 whereby the scrap receptacle can be moved to the scrap discharge position. Carriage 45 is fitted with a set of powered screw jacks 50 operable to lift the scrap receptacle 40 from a lowered position in which it is spaced below the wall enclosure 42 to a raised position in which it engages that section with the knife flange 49 penetrating the sand to form the seal 43 between the two.

[0051] Referring to FIG. 7, carriage 45 is moved along rails 47 by an electrically powered drive car 51 coupled to the carriage by a coupling 52. The carriage can move along the track 47 away from the caster to a transverse rail track 53 along which a pair of transverse rail carriages 54 are moveable by operation of appropriate drive means such as for example a pair of fluid cylinder units (not shown). The transverse carriages carry short rail track sections 60 which can be aligned with the rails 47 to allow the main carriage 45 selectively to be run onto them in order to transfer the scrap receptacle 40 from the caster via the track 47 to the transverse track 53. Scrap receptacle, illustratively though not necessarily filled with scrap can thus be moved onto one of the transverse carriages 54 and moved along the transverse track 53 to an unloading station 55 at which any scrap, collected in the scrap receptacle can be unloaded, for example, by an electromagnet hoist. At this time, a second, or any one of a number of replacement scrap receptacle(s) 40 held at a stand by position 58 can be moved along the transverse track 53 and run onto the main track 47 for movement to the lowered position beneath the caster 11, and can then be raised into engagement with the enclosure wall 42 thereby replacing the first scrap receptacle 40.

[0052] Sealed enclosure 10 further may comprise a wall section 61 disposed about the guide table 13 and connected to the frame of pinch roll stand 14 which includes a pair of pinch rolls 62 against which the enclosure is sealed by sliding seals 63.

[0053] Generally, the enclosure sections 41 and 61, together with the wall enclosure 42, may be lined with fire brick and the scrap receptacle(s) 40 may be lined for example either with fire brick or with a castable refractory lining.

[0054] The enclosure section 41 which surrounds the casting rolls 22 is formed with side plates 64 provided with notches 65 shaped to snugly receive the side dam plate holders 37 when the side dam plates 35 are pressed against the ends of the rolls by the cylinder units 36. The interfaces between the side plate holders 37 and the enclosure side wall sections 41 are sealed by sliding seals 66 to maintain sealing of the enclosure 10. Seals 66 may be formed of for example ceramic fiber rope.

[0055] The cylinder units 36 extend outwardly through the enclosure wall section 41 and at these locations the enclosure 10 is sealed by sealing plates 67 fitted to the cylinder units 36 so as to engage with the enclosure wall section 41 when the cylinder units are actuated to press the side plates against the ends of the rolls. Cylinder units 36 also move refractory slides 68 which are moved by the actuation of the cylinder units to close slots 69 in the top of the enclosure 10 through which the side plates are initially inserted into the enclosure and into the holders 37 for application to the rolls. The top of enclosure 10 is closed by the tundish 25, the side plate holders 37 and the slides 68 when the cylinder units are actuated to apply the side dam plates against the rolls. In this way, the complete enclosure 10 is sealed prior to a casting operation, thereby limiting the access of oxygen to the strip 12 as it passes from the casting rolls 22 to the pinch roll stand 14. Initially the strip will take up all of the oxygen from the enclosure space to form heavy scale on the strip. However, the sealing of the enclosure space controls the ingress of oxygen containing atmosphere below the amount of oxygen that could be taken up by the strip. Thus, after an initial start up period, the oxygen content in the sealed enclosure 10 will remain depleted thus limiting the availability of oxygen for oxidation of the strip. In this way, the formation of scale is controlled without the need to continuously feed a reducing or non oxidizing gas into the enclosure 10. It will be appreciated, however, that the feeding of a reducing or non-oxidizing gas into the enclosure 10 may be contemplated by the invention as well.

[0056] For example, in order to avoid the heavy scaling during the start up period, the enclosure 10 can be purged immediately prior to the commencement of casting so as to reduce the initial oxygen level within the enclosure 10 and so reduce the time for the oxygen level to be stabilized as a result of the interaction of oxygen from the sealed enclosure due to oxidation of the strip passing through it. The sealed enclosure 10 may conveniently be purged with nitrogen gas. It has been found that reduction of the initial oxygen content to levels of between 5% to 10% will limit the scaling of the strip at the exit from the enclosure to about 10 microns to 17 microns even during the initial start up phase. The breaking the seal between first scrap receptacle 40 and sealed enclosure wall 42 in change out of the scrap receptacle also allows the atmosphere within sealed enclosure 10 to be altered by, for example, the addition of controlled amounts of oxygen gas to the atmosphere.

[0057] The illustrated arrangement for moving and replacing the scrap receptacles 40 forming the lower part of the sealed enclosure has enabled an exchange or replacement procedure to be developed by which any scrap receptacle 40 can be replaced partway through a casting run and casting resumed after a short interruption to the cast without the need to completely terminate the casting of the strip. It has been found desirable to ensure that the interruption, if any, to the casting is no more than about five minutes and more desirably of no more than about 240 seconds duration in order to avoid thermal shock to the refractory components of the metal delivery system and the side dams when casting is continued or resumed and to ensure that steel does not solidify prematurely in the flow passages of the metal delivery system. In order to minimize the time interval during which casting is interrupted, the following exemplary procedure may be followed.

[0058] The accumulation of scrap in the scrap receptacle in the receiving position may be monitored by a scrap box operator at the scrap receiving station and/or remotely by an optical device such as a camera, a laser, a scale, or other suitable sensor(s). The operator and/or sensor(s) may also determine other reasons to replace the first scrap receptacle 40. In any event, due to myriad reasons, such as the receiving scrap receptacle 40 becoming filled with scrap, or for no particular reason at all, a decision may be made to replace the scrap receptacle in the scrap receiving position with any one of the other scrap receptacles 40 held at the stand by position. At this time, the scrap receptacle 40 may be lowered from the receiving position to a lowered position seating on a support carriage, breaking the seal 43 with enclosure wall 42. At generally the same time, the main caster operator may actuate slide gate 34 to interrupt the cast by interrupting the flow of molten steel to delivery vessel 26 while allowing casting to proceed for a time utilizing the molten steel already in delivery vessel 26 and nozzle 27 of the metal delivery system and the casting pool to continue the casting of the strip.

[0059] Although slide gate 34 may be operated at the same time or a little before downward movement of the scrap receptacle 40 is initiated, the cast interrupt interval will be minimized if lowering of the scrap receptacle 40 is initiated a little before the slide gate 34 is actuated to interrupt the casting of strip. It may typically take about 30 seconds to lower the scrap receptacle 40 from the receiving position by operation of the screw jacks 50 and the casting interrupt time will be minimized if the flow of molten steel to the metal delivery system is interrupted at or soon after the time that the this scrap receptacle 40 reaches its lowered position.

[0060] When the scrap receptacle 40 has reached its lowered position, a visual or other suitable check (e.g., a camera or sensor) may be used to ensure that scrap does not project upwardly so as to foul with the enclosure wall 42 when the scrap receptacle is moved. Some scrap may need to be pushed or dislodged to ensure appropriate clearance. It will be appreciated that the scrap receptacle 40 may be removed from the receiving position and replaced by any or the one or more scrap receptacle(s) 40 in the stand-by position(s) as described herein at any desired time. For example, it may be moved when empty, substantially empty, or substantially full of scrap, as desired. Moreover, as will be explained further, the replacement may take place with little or no interruption to casting of the strip.

[0061] As the flow of molten steel is interrupted to the metal delivery system, the scrap receptacle carriage drive car 51 is operated to drive the scrap receptacle carriage to the transverse track 53 and the carriage 45 is moved along the transverse track to the unloading station 55. The replacement scrap receptacle 40 is then brought across onto the main track 47 and brought into the lowered position beneath the upper enclosure wall 42. As soon as the replacement scrap receptacle 40 has reached this position, slide gate 34 is operated to continue of casting of strip by allowing the flow of molten steel to the distributor vessel 26 and the delivery nozzle 27 to initiate a normal casting start procedure. At substantially the same time, the scrap hoist jacks 50 are actuated so as to start lifting the replacement scrap receptacle 40 into its operative position as a new receiving scrap receptacle 40. By initiating the flow of metal to resume casting before the replacement scrap receptacle is actually raised into the operating position, time is saved without impairing safety since any molten metal or solidified material discharging during the start up process can still be caught in the scrap receptacle 40 in or near the receiving position.

[0062] As noted, the invention contemplates that the replacement procedure may be accomplished in certain instances as just described with little or no need to interrupt the casting of the strip. There may not even be a need to interrupt and restart the flow of molten steel via slide gate 34. For example, the casting need not be interrupted if there are no safety concerns over the discharge of imperfect material, such as for example where the replacement procedure is accomplished at a time and in a manner not susceptible to such discharge of scrap, or where the area below the scrap receptacle receiving area is configured to receive any discharged scrap in the event no scrap receptacle is then in the receiving position. Illustratively, such a time might be during steady-state casting, and such a manner may involve speed of receptacle replacement, which may be facilitated by the use of a plurality of receptacles as further described herein. Further illustratively, the area below the receiving area may be recessed and lined with fire brick or a refractory material.

[0063] In all instances, the illustrated apparatus and above described procedure have made it possible for scrap receptacle changes to be carried out during steady state casting, or alternatively, in about 240 to about 300 seconds, or less. It has been found that even if the flow of molten metal is interrupted, if the replacement procedure is carried out in about 240 seconds or less, then the refractories in the metal delivery system and the pool confinement plates retain sufficient heat to avoid thermal shock on resumption of casting and at the same time ensure that steel does not solidify prematurely in the flow passages of the metal delivery system. However, the specifically described apparatus and procedure is advanced by way of example only and could be modified. For example, it would be possible to have two scrap receptacles mounted on adjacent carriages hooked together to move in tandem along the main track so that when the first receptacle is ready to be changed out, it can be rapidly replaced with the second or subsequent receptacle(s) with minimal movement along the track. This would enable the change of scrap receptacle to be done in a much reduced time, after which it might be necessary to move both scrap receptacles to a discharge position. In another arrangement, a series of replacement scrap receptacles could be brought in from one side of the caster and the receiving scrap receptacle(s) moved off to an unloading station at the other side of the caster to avoid the need to move the outgoing receiving and incoming replacement receptacles along the same track section and so permit multiple changes with much reduced changeover intervals. In yet another configuration, one or more replacement receptacles may underlie the main track, with at least one of such replacement receptacles being positioned under the receiving position such that when the first scrap receptacle is lowered and moved out from under the receiving position, the second or subsequent receptacle is already positioned under the receiving area and ready to receive any scrap, even while being raised to the sealed configuration. Such configurations further exemplify how the replacement process may proceed with little or no interruption in casting of strip.

[0064] Although the invention has been illustrated and described in detail in the foregoing drawings and description with reference to several embodiments, it should be understood that the description is illustrative and not restrictive in character, and that the invention is not limited to the disclosed embodiments. Rather, the present invention covers all variations, modifications and equivalent structures that come within the scope and spirit of the invention. Additional features of the invention will become apparent to those skilled in the art upon consideration of the detailed description, which exemplifies the best mode of carrying out the invention as presently perceived. Many modifications may be made to the present invention as described above without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A method of producing thin cast steel strip by continuous casting comprising the steps of: a) assembling a pair of cooled casting rolls having a nip between them; b) introducing molten steel between the pair of casting rolls through a metal delivery system configured to a casting pool of molten metal supported on casting surfaces of the casting rolls; c) counter-rotating the casting rolls to form metal shells on the casting surfaces of the casting rolls; d) forming solidified thin steel strip through the nip between the casting rolls from said formed metal shells; e) discharging strip into a first scrap receptacle positioned in a scrap receiving position; f) replacing the first scrap receptacle with a replacement scrap receptacle by the steps comprising: (i) optionally, interrupting the flow of molten steel to the metal delivery system, (ii) moving the first scrap receptacle away from the scrap receiving position, (iii) positioning a replacement scrap receptacle into the scrap receiving position, and (iv) continuing or resuming the flow of molten steel to the metal delivery system to cast steel strip.
 2. The method described in claim 1 comprising the additional step of monitoring the status of the first scrap receptacle or replacement scrap receptacle in the scrap receiving position.
 3. The method described in claim 2 where a sensor is used to monitor the status of a scrap receptacle in the scrap receiving position.
 4. The method described in claim 3 where the sensor is an optical device.
 5. The method described in claim 3 where the sensor is a scale.
 6. The method described in claim 1 where the step of introducing molten steel between the pair of casting rolls comprises the step of positioning above the casting rolls a molten delivery system configured to introduce the molten steel between the pair of casting rolls, and. the step of replacing the first scrap receptacle with the replacement scrap receptacle comprises the steps of: i) interrupting the flow of molten steel to the metal delivery system; ii) moving the first receptacle away from the scrap receiving position, iii) bringing the replacement scrap receptacle into the scrap receiving position, and iv) resuming the flow of molten steel by the metal delivery system to the casting pool and continuing casting of steel strip.
 7. The method described in claim 6 wherein the flow of molten steel to the casting pool is interrupted by draining the casting pool.
 8. The method described in claim 1 where the step of replacing the first scrap receptacle with the replacement scrap receptacle comprises the steps of: i) continuing the flow of molten steel to the casting pool; ii) moving the first receptacle away from the scrap receiving position, iii) bringing the second scrap receptacle into the scrap receiving position, and iv) continuing casting of steel strip.
 9. The method described in claim 1 where the step of moving the first receptacle away from the scrap receiving position comprises the step of breaking a seal between the first scrap receptacle and a wall enclosure of an enclosure enclosing the pair of casting rolls.
 10. The method described in claim 6 where not more than about 5 minutes elapses between the steps of interrupting and resuming the flow of molten steel to the metal delivery system.
 11. The method described in claim 6 where not more than about 240 seconds elapses between the steps of interrupting and resuming the flow of molten steel to the metal delivery system.
 12. The method described in claim 6 further comprising the step of monitoring the status of the scrap in the first scrap receptacle or replacement scrap receptacle in the scrap receiving position.
 13. The method described in claim 6 further comprising the step of monitoring the status of the scrap in the first scrap receptacle or replacement scrap receptacle in the scrap receiving position.
 14. A twin roll caster apparatus for casting metal strip comprising: (a) a pair of generally horizontal casting rolls forming a nip between them, (b) a metal delivery system capable of delivering molten metal between the casting rolls to form a casting pool of molten metal supported on the casting rolls, (c) roll drivers to counter rotate the casting rolls thereby producing a cast strip delivered downwardly from the nip, (d) a sealed enclosure to receive the strip delivered from the nip formed with an opening in an enclosure wall, and a first moveable scrap receptacle capable of being positioned in sealing engagement with the enclosure wall around the opening, and (e) the enclosure being configured to allow replacement of the first moveable scrap receptacle by replacement moveable receptacle during casting operation by breaking the seal of the first scrap receptacle with the enclosure wall and thereafter making the seal by the replacement scrap receptacle with the enclosure wall.
 14. The twin roll caster described in claim 13 where the pair of casting rolls are disposed within an enclosure configured to form a seal with the first scrap receptacle or alternatively the replacement scrap receptacle when disposed in the scrap receiving position.
 15. The twin roll caster described in claim 13 further comprising an elevating mechanism configured to move the first scrap receptacle between a raised position and a lowered position.
 16. A twin roll caster apparatus comprising: (a) a pair of generally horizontal casting rolls forming a nip therebetween, (b) a metal delivery system comprising a delivery vessel and a delivery nozzle through which to deliver molten steel between the casting rolls above the nip to form a casting pool of molten steel supported on the casting rolls, (c) roll drivers to counter-rotate the casting rolls to produce a cast strip delivered downwardly from the nip, and (d) a sealed enclosure to receive the strip delivered from the nip having a wall enclosure in sealing engagement with a first moveable scrap receptacle configured to be replaced by a replacement moveable scrap receptacle during casting operations with little or no interruption in the casting of steel strip.
 17. The twin roll caster apparatus described in claim 16 where the pair of casting rolls are disposed within an enclosure configured to form a seal with the first scrap receptacle or alternatively the replacement scrap receptacle when disposed in the scrap receiving position.
 18. The twin roll caster apparatus described in claim 16 further comprising an elevating mechanism configured to move the first scrap receptacle between a raised position and a lowered position. 